Methods of soldering electronic parts include soldering with a soldering iron, flow soldering, reflow soldering, and the like.
Reflow soldering is a method in which a solder paste comprising a solder powder and a flux is supplied only to necessary locations of a printed circuit board by printing or with a dispenser, electronic parts are placed on the coated portions, and then the solder paste is melted in a heating apparatus such as a reflow furnace to solder the electronic parts to the printed circuit board. In the reflow method, not only is it possible to solder a large number of locations in a single operation, but bridging does not take place even if electronic parts are soldered with a narrow pitch and solder does not adhere to unnecessary locations. Therefore, soldering can be carried out with excellent productivity and reliability.
In the past, a Pb—Sn alloy was used in solder. This Pb—Sn alloy has a melting point of 183° C. for a eutectic composition (Pb-63Sn), so it has little thermal effect even on electronic parts having poor heat resistance. In addition, it has excellent solderability. Therefore, it has the advantages that it produces few soldering defects such as unsoldered portions or dewetting.
However, due to the problem of the toxicity of Pb, a strong demand has developed in recent years in the electronic equipment industry for so-called lead-free solder which does not contain Pb.
Lead-free solders which are currently widely used are ones having an Sn—Ag—Cu composition like that described in JP 5-050286 A, which contains 3-5 mass % of Ag and 0.5-3 mass % of Cu. Because this lead-free solder has superior heat cycle properties and creep properties compared to a conventional Sn—Pb solder, its use is spreading. Particularly heat cycle properties are an important factor in evaluation of the life span of electronic devices and in guaranteeing products.
Currently used lead-free solder alloys having an Sn—Ag—Cu composition are hard compared to conventionally used Sn—Pb solder. Therefore, when they are used in small devices such as mobile phones and if the devices are accidentally dropped, cracks develop in the interface between the electronic parts and solder joints, so these solders have the problem that so-called interfacial peeling easily occurs. Interfacial peeling does not readily occur with substrates soldered by flow soldering in which a relatively large amount of solder is used in solder joints. However, it easily occurs with substrates which are soldered by reflow soldering and which have minute joints formed using a small amount of solder.
Reflow soldering of substrates is performed using solder paste, solder balls, solder preforms, and the like. In addition, flux-cored solder is used to repair solder joints. Printed circuit boards in which these solder materials are used produce the problem of interfacial peeling particularly easily.
The present applicant disclosed a solder alloy having an Ag content of 0.8-2.0 mass %, a Cu content of 0.05-0.3 mass %, and further containing In, Ni, Pt, Sb, Bi, Fe, Al, and P as a solder alloy having good resistance to drop impact (resistance to impact due to dropping) for use in soldering to Cu lands (WO 2006/129713 A1).
As a solder alloy having excellent resistance to heat cycles, they also disclosed an Sn—Ag—Cu—Bi based lead-free solder which is an Sn—Ag—Cu based solder alloy containing solid solution elements and which has an alloy structure at room temperature comprising either a supersaturated solid solution or a solid solution in which solid solution elements are precipitated and which has an alloy structure at a high temperature in a cycle environment comprising a solid solution in which the solid solution elements precipitated at a low temperature are redissolved in the Sn matrix (WO 2009/011341 A1).
There is also disclosed a solder alloy in which Bi and Sb are added to an Sn—Ag—Cu solder composition such that Bi and Sb form solid solutions with Sn and Ag and Cu form intermetallic compounds with Sn, whereby mechanical strength is maintained by the microstructure formed from these solid solutions and intermetallic compounds (JP 9-327790 A).